1. Field of the Invention
The present invention relates to a mass spectrometer that is operated in a chop mode to substantially reduce the adverse effects of undesirable background signals on the operation of the mass spectrometer and, in particular, the effect of line signals on the operation of the mass spectrometer.
2. Description of the Related Art
Mass spectrometers are used to analyze the composition of a gas based upon the different masses of the components of the gas. For example, an appropriately configured mass spectrometer would identify nitrogen and oxygen as two of the constituents of an unidentified sample of air based on the difference in their masses, 28 amus and 32 amus, respectively. The typical mass spectrometer includes an evacuated chamber into which a sample of the gas to be analyzed is admitted. The evacuated chamber provides a vacuum environment that facilitates analysis of the molecular components of the gas sample based on their mass. To maintain the vacuum in the evacuated chamber, a vacuum pump, such as an ion pump, is employed. A valve or other regulating device is used to control the amount of gas admitted into the evacuated chamber so that the vacuum environment is not destroyed by exceeding the capacity of the vacuum pump. Located within the evacuated chamber are the instruments for analyzing the molecular components of the gas sample based on their mass and at least one detector for providing a signal indicative of the presence or absence of a particular type of molecule in the gas sample. Processing circuitry, such as amplifiers and filters, are employed to place the signal output by the detector in a meaningful form.
French Pat. No. 2,562,323, which issued to S.a.r.1. Laboratoire d'Etude des Surfaces, published on Oct. 4, 1985, and is entitled "Process for Simultaneous Analysis of Several Streams of Gas by Mass Spectrometry, and Operation Thereof", identifies gasses that have accumulated on the walls of the evacuated chamber and in the pumping system as sources of undesirable background signals that can adversely affect the signal to noise ratio of the signal output by the mass spectrometer. To reduce the adverse effects of these undesirable signals, the French patent proposes operating the mass spectrometer in a chop mode where the gas to be analyzed is admitted to the evacuated chamber at a particular frequency by pulsing the valve and the signal output by the evacuated chamber is detected at the same frequency. The patent further indicates that the detection can be accomplished numerically with a microprocessor. Providing the frequencies of the undesirable background signals are substantially less than the frequency at which the gas samples are being admitted to the evacuated chamber, the chop mode of operation substantially isolates the background signals from the signal of interest. More specifically, the chop mode of operation results in an output signal where the background signals are substantially isolated in the DC component of the output signal and the signal of interest, the component of the output signal corresponding to the frequency at which the valve is being driven, is in the AC component of the signal. Due to this isolation, the effects of the undesirable background signals are reduced and the signal to noise ratio is improved.
Unrecognized, however, by the French patent is the undesirable effect that the line signal, the 60 Hz signal that is used in the U.S. to provide the mass spectrometer or machinery located adjacent to the mass spectrometer with power, has on the output signal. To effectively reduce the effects of the line signal on the output signal of the mass spectrometer, the valve would have to be chopped at a frequency of several hundred hertz. This, however, is generally not practicable because the passage within the valve and the evacuated chamber through which the gas sample must flow does not have a sufficient bandwidth to accommodate such a high chop rate. Moreover, a high chop rate requires a corresponding increase in the speed and complexity of the circuitry used to process the signals.
Based on the foregoing, there is a need for a chop mode mass spectrometer that is capable of reducing the effect of line signals on the operation of the mass spectrometer.